In the mammalian auditory system, medial olivocochlear (MOC) efferent fibers are known to modify basilar membrane mechanics via synaptic terminations on the outer hair cells (OHCs), but the functional significance of this action is still being elucidated. The broad objective of the proposed study is to gain increased understanding of how the MOC modifies the mechanics of the cochlea. While many studies have measured the effects of MOC stimulation using distortion product otoacoustic emissions, very few studies have used stimulus frequency OAEs (SFOAEs), which are potentially a more accurate probe of MOC function. Two specific aims are considered in relation to the broad objective. 1) The frequency tuning of the MOC reflex will be examined using SFOAE probe tones in conjunction with various tonal and bandpass noise elicitors. 2) The effects of the MOC reflex on the OHC nonlinearity will be examined using amplitude-modulated SFOAEs. In both aims, data will be collected in humans and guinea pigs. In addition to providing valuable cross-species comparisons, the animals will provide an important control to rule out non-MOC effects. Understanding MOC effects on cochlear mechanics is important for understanding the function of the efferent system in hearing. The effect of MOC activation on OAE measurement has important clinical applications for the identification of hearing loss and auditory neuropathy.